Syntrophus Aciditrophicus (Sa) lives a syntrophic lifestyle, surviving by sharing metabolites with other species within microbial communities. Microbial communities are important to human and environmental health (e.g. infectious biofilms and bioremediation). Despite this, most proteomic studies of microbes involve mono, cultures, with little attention paid to interspecies interaction. Our work will be the first to use proteomics to study microbial cooperation. We are interested in the metabolic pathways and protein diversification strategies Sa has adopted for syntrophy. We will study the effect of interaction with a syntrophic partner, Methanospirillum hungatei (Mh), on Sa's proteome, using gel-based separation and mass spectrometry. To gain insight into how Sa's proteome adjusts to reflect changing energetic demands resulting from interspecies interactions, the protein profiles Sa grown in mono and cocultures with Mh will be compared. In addition to mapping protein networks, special attention will be paid to Sa's enigmatic protein processing. This includes, N-terminal modification heterogeneity, and variant protein production by receding and/or miscoding. Receding is of particularly importance to human health, as viruses use it extensively (including HIV). Protein modifications are very difficult to study due to extreme complexity and the lack of sensitive tools. We have developed a novel technology for identifying protein modifications and variant proteins. This technology includes mass-analyzing intact proteins directly from IEF gels. Precise protein masses correlated to identified 2D gel spots will reveal modified and variant protein forms. Ratios of variant products and N-terminally modified populations will be monitored as culture conditions change. Successful development of this technology would impact essentially all biological endeavors with proteins.